Butterfly valves are used in many nuclear steam turbine applications, particularly at lower pressures than for fossil fuel turbines. Some manufacturers have now developed tight seating valves (i.e., without leakage when closed), and valves for higher pressures and temperatures than used previously. These developments have lead to the possibility of applying this type of valve as an inlet valve in steam turbines.
One of the characteristics of a butterfly valve is that it creates a fairly strong "wake" downstream in a fully open as well as in a partially closed position. This wake is often described in the literature as a "Von Karman vortex alley", so named for the Hungarian/German aerodynamicist who has described it. The wake behind a butterfly valve will subside slowly if a long run of pipe is provided down stream. In turbine valve applications, however, such a long run is generally not practical as it will leave a large volume of steam downstream of the valve which may contribute to control problems, for example, inadequate limitation of overspeed. On the other hand, if the butterfly valve is mounted close to the steam turbine inlet, the flow disturbances of the wake will enter into the first stage of the turbine where they may excite the first stage diaphragm, stationary and rotating blades, possibly causing damaging cyclic stresses in these elements. Therefore, a successful application of a butterfly valve as controlling valve for a steam turbine requires a means to protect the turbine first stage from the effects of the wake behind the valve. A "flow straightener" is a possible means for dissipating the wake, but it also requires a fairly long length of pipe and adds unwanted cost.